This proposal seeks support to study the structural basis of specific macromolecular interaction typical of the cell surface. The core of the experimental approach will be X-ray diffraction studies of single crystals. The systems to be examined by this approach are (1) Neurophysin-vasopressin complex, a complex between a protein and peptide hormone; (2) delta 5-3-ketosteroid isomerase, a protein that interacts specifically with a steroid hormone; (3) snake venom phosphodiesterase, a protein that recognizes specific phospholipids (crystalline); (4) Non-covalent complex between a lytic fragment of "band 3" protein (human RBC ghosts) and mammalian enzymes; (5) the cholera toxin. In all cases but (4) crystals of these materials have been obtained. BIBLIOGRAPHIC REFERENCES: Pasek, M., Keith, C., Feldman, D. and Sigler, P.B. Characterization of crystals of two venom phosphopholipases A2. J. Mol. Biol. 97, 395-397 (1975). Westbrook, E.M., Sigler, P.B., Berman, H., Glusker, J.P., Bunick, G., Benson, A. and Talalay, P. Characterization of a monoclinic crystal form of an enzyme of steroid metabolism, delta 5-3-ketosteroid isomerase. J. Mol. Biol. 103, 665-667. (1976).